There are two main sources of the Earth's internal heat, the primordial heat, and the radioactive heat. Based on a study conducted in Japan by Itaru Shimizu, a particle physicist at Tohoku University, and his colleagues, 54% of the Earth's internal heat reaching the Earth's surface is from radioactive heat. The rest is from primordial heat.
A. Primordial Heat
- This is an accumulated heat released or formed during the early stages of the Earth’s formation. There are two categories of primordial heat based on the source and period of heat production. These are accretional heat and gravitational release.
a. Accretional Heat
This was produced during the accretion stage of the Earth's formation. In this early stage, atoms and particles were sticking together, forming larger particles. As these larger particles gently collide with each other, they started to create more massive balls until a planetesimal was formed. Planetesimals are large or massive enough to possess gravity. Its gravity causes it to attract smaller objects. At this point, the planetesimal was being bombarded by comets, asteroids, etc. Influenced by other processes, this planetesimal evolved to a planet. The accretional heat was released during particle collisions and Earth's planetesimal or protoplanet bombardment.
How?
During the collision process, the Earth heats up because of the kinetic energy of the colliding bodies. This kinetic energy is converted to heat. If two meteorites collide, the heat radiates back into space. But if the collision involves a planetesimal, the energy deeply penetrates that allows heating beneath the planetesimals surface. The debris during the collision blankets or covers the planetesimals surface, retaining the heat inside of it. The amount or percentage of heat release during accretion is unknown because of factors such as it being dependent on environmental conditions and specific formation timescales.
* The scenario is similar: hitting a hammer on a hard surface. After the hitting, the hammer heats up. The kinetic energy is transformed to heat energy.
b. Gravitational Release
During the formation of planetesimals and protoplanet (differentiation process), the gravitational potential of dense materials (metals) is converted to heat. In this process, the materials that compose a planetesimal or protoplanet separates according to their density. Materials or components with the highest density (iron) are moved at the center or core, while materials with the lowest density are moved at the surface. This movement of materials or components with high density creates friction and releases heat (1,000 K (1,800 °F; 1,000 °C). The amount of heat energy released is explained by the formula (E = - G M m / r) where:
E= Energy; G= Gravitation force; M and m = Masses of the two involved objects; and r= distance from the center
B. Radioactive Heat
These are heat produced from four radioactive isotopes found in the Earth’s core or mantle. These are the K40, Th232, U235, and U238.
Potassium-40 (K40) has a half-life of 1.28 × 109 years. Thorium-232 has a half-life of about 14 billion years. Uranium-235 half-life is over 700 million years while U238 has over 4.5 billion years. Thus this radioactive isotopes can supply parts of our Earth’s internal heat for billions of years.
Estimated Earth’s Internal Temperature
Inner Core: 9,000 to 13,000 degrees Fahrenheit (5,000 to 7,000 degrees Celsius)
Outer Core: 7,200 to 9,000 degrees Fahrenheit (4,000 to 5,000 degrees Celsius)
Mantle: 1,000° Celsius (1,832° Fahrenheit) near its boundary with the crust, to 3,700° Celsius (6,692° Fahrenheit) near its boundary with the core
EARTH’S REDISTRIBUTION OF HEAT
a. Convection
- This is the transfer of heat as liquid and solid materials move due to density and temperature differences. This happens within the mantle and outer core as denser or colder liquid material sink while hotter or lighter liquid material rises.
b. Conduction
- It is the transfer of heat between solid and gas or liquid and solid. This happens from the core to the mantle and from the asthenosphere to the crust.
c. Radiation
- It is the transfer of heat from the crust to things on the Earth's surface through the atmosphere or electromagnetic waves.
d. Advection
- It is the horizontal transfer of heat through fluid flow. Its difference with convection is that this is not dependent on density differences, but an external or outside force is needed for the transfer to occur. Examples of these outside forces are wind or currents.
References:
https://opentextbc.ca/geology/chapter/3-2-magma-and-magma-formation/
https://www.geol.umd.edu/~jmerck/geol212/lectures/10.html
http://astronomy.nmsu.edu/kurt/Astronomy110G/Lectures/18.Origins-I.pdf
https://www.britannica.com/place/Earth/Effects-of-planetesimal-impacts
https://physics.info/gravitation-energy/
https://earthsky.org/earth/what-is-the-source-of-the-heat-in-the-earths-interior
https://www.livescience.com/15084-radioactive-decay-increases-earths-heat.html
https://www.atsdr.cdc.gov/phs/phs.asp?id=658&tid=121
https://www.sciencemag.org/news/2011/07/earth-still-retains-much-its-original-heat